Field of the Invention
The present invention relates to a dual-chambered molten metal holding furnace for low pressure casting that is preferable for producing cast products such as aluminum alloys by means of the low pressure casting method.
Description of the Related Art
A dual-chambered molten metal holding furnace for low pressure casting including a molten metal storage container that partitions a molten metal holding chamber and a pressurizing chamber on the inside thereof and is formed of a monolithic refractory material; a cover plate made of a steel product that covers a bottom surface, a side surface, and a top surface of the molten metal storage container via a heat insulation layer and/or a fireproof layer around its circumference; a molten metal passage opening that is provided between the molten metal holding chamber and the pressurizing chamber; a lifting cutoff valve that opens and closes the molten metal passage opening; and tube heaters that are placed, respectively, in the interior of the molten metal holding chamber and the interior of the pressurizing chamber, wherein the pressurizing chamber includes a pressurizing part and a molten metal output part which are communicated with each other at the bottom thereof, a pressurizing pipe and a molten metal output pipe, each of which is a heat-resistant integral sintered product having impermeability and molded from fine ceramics or the like, are mounted inside the pressurizing part and the molten metal output part, respectively, is known (see Patent Document 1 with the applicant being the same as the present applicant). Although the monolithic refractory material constituting the molten metal storage container is permeable, an upper space of the molten metal surface level in the pressurizing chamber is in a completely sealed structure by the impermeable pressurizing pipe so as to take measures for seeping the molten metal into the molten metal storage container.
A heat-resistant integral sintered product molded from a material having permeability to some extent has been employed as each of the pressurizing pipe and the molten metal output pipe. In this case, since the pressurizing pipe and the molten metal output pipe exhibit a slight permeability, pressurized gas enters a material constituting the molten metal storage container from the pressurizing pipe. After the pressurized gas is held in the material for some time, the pressurized gas is released again in the molten metal, resulting in generation of air bubbles in the molten metal and defective products. Thus, in the technique disclosed in Patent Document 1, impermeability obtained by employing fine ceramics or the like which may be very expensive as a material is intended to prevent entry of pressurized gas into the material constituting the molten metal storage container from the pressurizing pipe and generation of air bubbles in the molten metal in association with reemission of the pressurized gas.